Tin-indium based lead-free solders with zinc addition

ABSTRACT

A new kind of Sn—In based Pb-free solders with Zn addition is disclosed, which includes: 15˜25 wt % In; 0.05˜1.5 wt % Zn; and balance Sn. When the solder of the present invention is used in the assembly of electrical products, the dissolution rates of the substrates and the growth of the intermetallic compounds formed at the interfaces can be reduced; and thereby the properties of joints can be improved.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lead-free solder and, more particularly, to a tin-indium based lead-free solder with the addition of zinc, which is used as a soldering material.

2. Description of Related Art

In assembly of electrical products, solders are used for jointing various electronic components to a printed circuit board, in which soldering is one of the most important connection technologies.

Regarding soldering materials, metals with a low melting point and good ductility are considered. Traditionally, tin-lead alloys are widely used as solders. Currently, based on RoHS lead-free directive, many researchers have devoted themselves to the development of lead-free solders. Basically, lead-free solders contain tin as the main component and other alloy elements. In addition to presently popular Sn—Ag—Cu alloys, other alloys (such as Sn—Cu alloys and Sn—In alloys) also can be used as lead-free solders.

In soldering, solders with a lower melting point are first melted and then connected with a substrate, in which atom diffusion between the molten solders and the substrate occurs and thus a layer of intermetallic compounds is formed at the interface. However, if the layer of intermetallic compounds is too large in thickness, the mechanical properties of joints will be bad and peeling easily occurs. Besides, in soldering, the undercooling of molten solders will result in change of curing order and thereby the microstructure of joints and the soldering process are badly affected. Accordingly, the properties of joints are associated with undercooling of solders, dissolution of the substrate and the growth of the intermetallic phase.

Thereby, it is desirable to develop a lead-free solder consisting of fewer elements, resulting in an easier manufacturing process, easily anticipated diffusion pathway of atoms, optimized properties of joints and enhanced reliability of electronic products.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a lead-free solder, in which less undercooling, lower substrate dissolution rates, lower growth rates of intermetallic compounds formed at interfaces and thus improved properties of joints can be achieved.

To achieve the object, the present invention provides a lead-free solder, including: indium of 15-25 wt %, zinc of 0.05-1.5 wt %, and balance tin. Herein, “balance tin” refers to a certain amount of tin where the lead-free solder reaches 100 wt % in total.

In the present invention, a small amount of zinc is added into Sn—In-based alloys with a low melting point and good wettability, and thereby the lead-free solder including Sn—In—Zn alloys as main components is provided. Accordingly, the cost can be reduced and the drawback with regard to the difficulty to obtain indium can be mitigated. Meanwhile, by the addition of zinc in a small amount, the lead-free solder according to the present invention can achieve less undercooling, lower substrate dissolution rates, and lower growth rates of intermetallic compounds formed at the interface. Besides, in comparison to quaternary or pentanary alloys commonly used on the market, the lead-free solder according to the present invention may merely include three metal elements, and thereby the process for preparing the same is easier.

In the lead-free solder according to the present invention, a first metal of 0.01-2.0 wt % may be further included, where the first metal may be selected from the group consisting of Sb, Bi, Ge, Fe, Al, Ag, Cu, Ce and La.

In the lead-free solder according to the present invention, a second metal of 0.01-2.0 wt % may be further included, where the second metal may be selected from the group consisting of Sb, Bi, Ge, Fe, Al, Ag, Cu, Ce and La.

In addition, the lead-free solder according to the present invention may further include other impurities and additives in a trace amount.

Preferably, the lead-free solder according to the present invention includes: indium of 15-25 wt %, zinc of 0.05-1.5 wt %, and tin of 73.5-84.95 wt %. More preferably, the lead-free solder according to the present invention includes: indium of 20 wt %, zinc of 0.05-1.5 wt %, and tin of 78.5-79.95 wt %. Most preferably, the lead-free solder according to the present invention includes: indium of 20 wt %, zinc of 0.5-1.0 wt %, and tin of 79-79.5 wt %.

In one aspect of the present invention, the lead-free solder includes: indium of 20 wt %, zinc of 0.5 wt %, and tin of 79.5 wt %.

In another aspect of the present invention, the lead-free solder includes: indium of 20 wt %, zinc of 0.7 wt %, and tin of 79.3 wt %.

In yet another aspect of the present invention, the lead-free solder includes: indium of 20 wt %, zinc of 1.0 wt %, and tin of 79.0 wt %.

Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an SEM image of the Pb-free solder according to Example 1 of the present invention on a nickel substrate;

FIG. 2 is an SEM image of the Pb-free solder according to Example 2 of the present invention on a nickel substrate;

FIG. 3 is an SEM image of the Pb-free solder according to Example 3 of the present invention on a nickel substrate;

FIG. 4 is an SEM image of the Pb-free solder according to Comparative Example of the present invention on a nickel substrate; and

FIG. 5 is a diagram for showing the dissolution amount of the substrate in the Pb-free solders according to Examples 1-3 and Comparative Example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Preparation of Pb-Free Solders

According to Table 1, Pb-free solders were prepared in the Examples and Comparative Example. Herein, the amount of each component in Pb-free solders was represented in wt %.

TABLE 1 Components (wt%) Tin Indium Zinc Aluminum Silver (Sn) (In) (Zn) (Al) (Ag) Example 1 79.5 20 0.5 — — Example 2 79.3 20 0.7 — — Example 3 79.0 20 1.0 — — Example 4 83.5 15 1.0 0.5 — Example 5 72 25 1.5 0.5 1.0 Comparative 80 20 — — — Example

These above-mentioned metal elements were mixed and melted by heating, followed by cooling, so as to obtain Pb-free solders according to the Examples and Comparative Example.

Test for Properties of Pb-Free Solders

The Pb-free solders according to Examples 1-3 and Comparative Example were taken as 7 mg, and a differential scanning calorimet (DSC) was used for testing the properties of Pb-free solders. These Pb-free solders were heated up to 300° C. at 5° C./min and maintained at the temperature for 5 minutes, and then cooled to room temperature at 5° C./min. These measured liquidus temperature, solidus temperature and beginning curing temperature are shown in Table 2.

TABLE 2 Beginning Solidus Liquidus Curing Degree of Temperature Temperature Temperature Undercooling (° C.) (° C.) (° C.) (° C.) Comparative 155 196 178 18 Example Example 1 136 193 191 2 Example 2 132 194 193 1 Example 3 142 193 192 1

Undercooling affects the phase of solders during curing process, and thereby the manufacturing process and reliability of products are associated with degree of undercooling. From Table 2, it can be confirmed that the Sn—In based Pb-free solders with Zn addition (Examples 1-3) indeed exhibit less undercooling than the Sn—In based Pb-free solders without Zn addition (Comparative Example), and thereby improved reliability of products is achieved.

Test for Growth of Intermetallic Phase

These Pb-free solders according to Examples 1-3 and Comparative Example were taken as 2 g, placed on a pure Ni substrate and sealed in a quartz tube of 6 mm/8 mm in inside diameter/outside diameter. Subsequently, the Ni substrate with these Pb-free solders disposed thereon was placed in an oven at 230° C. for 2 hours, followed by quenching. Finally, the interfaces were observed by means of a scanning electron microscope (SEM). FIGS. 1-3 show SEM images of the Pb-free solders of Examples 1-3 on the Ni substrate, respectively; and FIG. 4 shows an SEM image of the Pb-free solder of Comparative Example on the Ni substrate, at a magnification factor of 500 times.

From these SEM images shown in FIGS. 1-4, it can be confirmed that in comparison to the Sn—In based Pb-free solder without the addition of Zn (Comparative Example), the amount of the intermetallic phase (i.e. Ni₃Sn₄) diffusing into the solders is smaller in the case of the Sn—In based Pb-free solders with the addition of Zn (Examples 1-3). That is, the growth rate of the intermetallic phase significantly decreases when using the Sn—In based Pb-free solders with the addition of Zn (Examples 1-3).

If the thickness of the intermetallic phase is too large, the mechanical properties of joints will be reduced and thereby peeling or cracking occurs. Accordingly, since the Sn—In based Pb-free solders with the addition of Zn according to the present invention indeed exhibit lower growth rate of the intermetallic phase, the mechanical properties of joints can be improved, resulting in enhanced reliability of products.

Test for Dissolution of Substrate in Solders

These Pb-free solders according to Examples 1-3 and Comparative Example were taken as 5 g and placed in an oven at 230° C. Then, silver wires of 2 mm in diameter (1000 μm in radius) were vertically inserted into the molten solders for 10, 30, 50 and 90 minutes, followed by quenching. Finally, the change in the sectional area of the silver wire was observed by means of a scanning electron microscope (SEM). The results are shown in Table 3 and FIG. 5.

TABLE 3 Time 10 minutes 30 minutes 50 minutes 90 minutes Radium of Silver Wire (μm) Example 1 889 757 753 727 Example 2 886 767 757 742 Example 3 917 833 792 783 Comparative Example 842 721 696 667

Table 3 and FIG. 5 suggest that the dissolution amount of the silver wire in the Sn—In based Pb-free solders with the addition of Zn (Examples 1-3) is indeed smaller in comparison to the Sn—In based Pb-free solder without the addition of Zn (Comparative Example). Since silver is usually used in the passivation layer on the surface of the copper substrate, the dissolution of the silver passivation layer on the surface of the copper substrate can be reduced and thereby the possibility of contact between solders and the copper substrate can be reduced when using the Sn—In based Pb-free solders with the addition of Zn according to the present invention.

In the present invention, less undercooling, lower substrate dissolution and lower growth rate of the intermetallic phase indeed can be achieved when using the Sn—In based Pb-free solders with the addition of Zn according to the present invention. Thereby, the properties of joints in the assembly of electrical products can be improved by using the Sn—In based Pb-free alloys with the addition of Zn according to the present invention as soldering materials, resulting in enhanced reliability of electrical products.

Although the present invention has been explained in relation to its preferred embodiment, it is to be understood that many other possible modifications and variations can be made without departing from the scope of the invention as hereinafter claimed. 

1. A lead-free solder, comprising: indium of 15-25 wt %; zinc of 0.05-1.5 wt %; and balance tin.
 2. The lead-free solder as claimed in claim 1, further comprising: a first metal of 0.01-2.0 wt %, selected from the group consisting of Sb, Bi, Ge, Fe, Al, Ag, Cu, Ce and La.
 3. The lead-free solder as claimed in claim 2, further comprising: a second metal of 0.01-2.0 wt %, selected from the group consisting of Sb, Bi, Ge, Fe, Al, Ag, Cu, Ce and La.
 4. The lead-free solder as claimed in claim 1, comprising: indium of 15-25 wt %, zinc of 0.05-1.5 wt %, and tin of 73.5-84.95 wt %.
 5. The lead-free solder as claimed in claim 1, comprising: indium of 20 wt %, zinc of 0.05-1.5 wt %, and tin of 78.5-79.95 wt %.
 6. The lead-free solder as claimed in claim 1, comprising: indium of 20 wt %, zinc of 0.5-1.0 wt %, and tin of 79-79.5 wt %.
 7. The lead-free solder as claimed in claim 1, comprising: indium of 20 wt %, zinc of 0.5 wt %, and tin of 79.5 wt %.
 8. The lead-free solder as claimed in claim 1, comprising: indium of 20 wt %, zinc of 0.7 wt %, and tin of 79.3 wt %.
 9. The lead-free solder as claimed in claim 1, comprising: indium of 20 wt %, zinc of 1.0 wt %, and tin of 79.0 wt %. 